Little John, the difference in cross sectional area between a single 1.25" line (normal ID of around 1") and two 1" lines (normal ID of around 3/4" each) is less than 0.1 square inches in favor of dual lines. The friction loss of more water touching the inner walls in two lines coupled with the additional fittings required to bring them back into one line completely negates any advantage of an additional 0.1 square inches of additional space and probably is more detrimental than sticking with a single 1.25" line.
An advantage to dual lines, however, would be if you wanted to have two separate flows in the case of running two different buildings or appliances that would cause such a large drop in water temperature that you couldn't run that same water through a second appliance effectively. It would also be an advantage if you wanted an emergency backup setup where one line runs your heat and the other runs your water heater, etc. with each line on it's own pump. If the heat line failed, you could shut it down, do some quick simple plumbing inside of the house, and have the remaining line running your heat again.
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Topic: 1 1/4" VS two 1" Pex runs (Read 4576 times)